The present invention relates to a developing apparatus to be used in image forming apparatuses such as a copier and laser beam printer which use electrophotographic systems and electrostatic recording systems for visualizing latent images formed on image bearing members by allowing toners in two-component developer to visualize the latent images. Specifically, the present invention relates to a developing apparatus which has an improved developer carrying member to be used for conveying a developer.
A developing apparatus to be used in an image forming apparatus such as a copier or a printer which uses an electrophotographic system for visualizing an electrostatic latent image formed on an image bearing member such as a photosensitive drum by allowing a toner in a developer to adhere to the latent image uses a developing sleeve (developer carrying member) such as a metallic developing sleeve bears a developer contained in a developer container on the developing sleeve, conveys the developer to a developing area opposed to the image bearing member and develops the electrostatic latent image formed on the image bearing member with the toner in the developer, thereby visualizing the latent image.
Used as the toner is a magnetic one-component toner consisting of a magnetic toner, non-magnetic one-component developer consisting of a non-magnetic toner or a two-component developer containing a non-magnetic toner and a magnetic carrier, and a material and a shape of the developing sleeve are selected dependently on the developer. In case of the two-component developer, a developing sleeve having magnetic field producing means such as a magnet provided therein is used, and a non-magnetic metal such as stainless steel or aluminium has conventionally been used mainly as a material of the developing sleeve.
In a developing apparatus such as that described above, a property to convey the two-component developer consisting of the non-magnetic toner (hereinafter referred to simply as xe2x80x9ctonerxe2x80x9d) and the magnetic carrier (hereinafter referred to simply as xe2x80x9ccarrierxe2x80x9d) to the developing area is enhanced by roughening a surface of the developing sleeve and a developer layer can be coated uniformly over the surface of the developing sleeve.
As a method to roughen the surface of the developing sleeve, there have been proposed and carried out a sandpaper method for rubbing the surface of the developing sleeve with sandpaper, a sand-blast method using particles having indeterminate forms, a mixture method combining these methods, a chemical etching method utilizing a chemical treatment and the like.
However, the conventional developing sleeve has problems which are described below.
When the developing sleeve which has a surface roughened by any one of the methods is used for a long time, the toner or a component of the toner tends to be caught and adhere in and to valleys (concavities) out of convexities and concavities on the roughened surface. When the developing sleeve is used for a long time, the toner adhering to the valleys tends to be fused by frictional heat generated by a pressed layer thickness regulating member which regulates a developer latent thickness on a sleeve surface layer and may contaminate the surface of the developing sleeve.
When the two-component developer containing the carrier is used, if there are the concavities and convexities on the surface of the developing sleeve, the toner or the component of the toner tends to be embedded into the valleys (narrow valleys in particular) by carrier""s pressure. The toner embedded into the valleys fused to the developing sleeve, thereby tending to contaminate the surface of the developing sleeve after the developing sleeve is used for a long time.
In recent years where demands for color copiers and color printers have been enhanced, a particle size of a toner has been reduced and a softening point of the toner has been lowered to meet demands for higher qualities of images provided by copiers and printers as well as lower power consumptions, whereby the toner or a component in the toner has a higher tendency to be fused to the concavities and convexities on surfaces of developing sleeves which are roughened by the above described methods, resulting in contamination.
When the toner is fused to the surface of the developing sleeve, a developer is conveyed to a developing area in a smaller amount, thereby lowering an optical density of an image. Furthermore, it is conventional to apply a developing voltage of a DC voltage and/or an AC voltage to the developing sleeve for favorable development at a developing time, but when the toner is fused to the surface of the developing sleeve, a fused matter forms a high resistance layer on the surface of the developing sleeve, thereby hindering a desired electric field in the developing area between the developing sleeve and the image bearing body at a developing time.
An influence due to resistance enhanced by the contamination is remarkable in particular in a compact developing system which is adopted to form a high quality image using the two-component developer and capable of obtaining a sufficient optical density by reserving a distance of 1 mm or shorter between a developing sleeve and a photosensitive drum, and carrying out development while allowing a toner to fly from a surface of the developing sleeve using an electric field which is produced between the developing sleeve and the photosensitive drum by applying a DC voltage overlapped with an AC voltage. As a result, the enhanced resistance tends to make it impossible to obtain a sufficient developing effect of the developing bias voltage for the toner on a surface layer of the developing sleeve, thereby lowering an optical density or producing an image defect such as a white blank.
Actual image evaluations which were effected using a developing sleeve which was not contaminated, a developing sleeve which was contaminated and a developer provided a result that the contaminated developing sleeve lowered an optical density of an image 0.2 as compared with the developing sleeve which was not contaminated and produced an image defect such as the white blank. It will be understood from this result that contamination of a developing sleeve surface by a toner fused to the developing sleeve surface apparently constitutes a cause for lowering of an optical density and an image defect.
An object of the present invention is to solve the above described problem and provide a developing apparatus which prevents a toner from being embedded into a surface of a developer carrying member by a carrier when a two-component developer is used and free from contamination of the developer carrying member by a fused toner, thereby being making it possible to obtain good quality images stably for a long time.
An object of the present invention is to provide a developing apparatus for an image forming apparatus comprising means for forming an electrostatic latent image corresponding to image information on an image bearing member comprising:
a developer bearing body which bears and conveys a two-component developer containing a non-magnetic toner and a magnetic carrier to a developing area for forming a toner image by developing the electrostatic latent image on the above described image bearing member,
wherein convexities and concavities are formed on a surface of the above described developer carrying member and an average peak-to-peak spacing (Sm) is set at ⅓ to 6 times of a weight-average particle diameter D of the above described magnetic carrier (D/3xe2x89xa6Smxe2x89xa66*D), wherein average roughness at ten points (Rz) on the surface of the above described developer carrying member is set at {fraction (1/10)} to xc2xd times of a weight-average particle diameter (D) of the magnetic carrier and wherein concavities which are 1 xcexcm to 10 xcexcm wide by 0.2 xcexcm or deeper exist in a number smaller than 10 within a spacing of 100 xcexcm in a surface profile of the above described developer carrying member.